1. Field of the Invention
The present invention relates to a synchronous-rectification step-up DC/DC (direct-current to direct-current) converter, and also relates to an electronic appliance incorporating such a DC/DC converter.
2. Description of Related Art
Conventionally, as a type of means for supplying stabilized electric power with little heat loss and with comparatively high efficiency when there is a large input-output difference, there have been widely used step-up DC/DC converters (so-called switching regulators) that produce a desired output voltage from an input voltage by driving an energy storage element (such as a capacitor or inductor) through the switching of an output transistor (through the control of its duty).
In particular, in switching regulators that are required to offer high conversion efficiency, with a view to minimizing the ON resistance of a rectification element, as shown in FIG. 4A, a synchronous-rectification transistor P1 is used as a rectification element, and it is switched complementarily with respect to an output transistor N1.
As a conventional technique (for reducing power loss at switching) related to the foregoing, JP-A-2005-160198 discloses and proposes a step-up DC/DC converter in which, as shown in FIG. 4B, the gate voltage of an output transistor N1 is swung between a ground voltage GND and an input voltage Vin and meanwhile the gate of a synchronous-rectification transistor P1 is swung between the input voltage Vin and an output voltage Vout.
Certainly, with the synchronous-rectification step-up DC/DC converter shown in FIG. 4A, it is possible to minimize the ON resistance of the rectification element, and thereby to obtain higher conversion efficiency.
Disadvantageously, however, in the step-up DC/DC converter shown in FIG. 4A, a P-channel field-effect transistor is used as the synchronous-rectification transistor P1, and in addition the synchronous-rectification transistor P1 is driven as a result of its gate voltage being made to pulsate between the ground voltage GND and the output voltage Vout by a second driver DRV2 that receives the output voltage Vout as a positive supply voltage; consequently, in the step-up DC/DC converter shown in FIG. 4A, as compared with a first driver DRV1 that receives the input voltage Vin as a positive supply voltage, the second driver DRV2 that receives the output voltage Vout as a positive supply voltage produces more power loss (resulting in increased power loss in the entire step-up DC/DC converter).
On the other hand, certainly, with the step-up DC/DC converter shown in FIG. 4B, where the drive current of the second driver DRV2 is fed back to the input terminal, it is possible to overcome the disadvantage mentioned above.
Inconveniently, however, in the step-up DC/DC converter shown in FIG. 4B, in case a fall or variation in the input voltage Vin (as results from a battery) is such that the potential difference between the input voltage Vin and the output voltage Vout is smaller than the ON threshold voltage of the synchronous-rectification transistor P1, it is no longer possible to turn the synchronous-rectification transistor P1 ON.